Embodiments disclosed herein relate generally to processes for upgading petroleum feedstocks. In one aspect, embodiments disclosed herein relate to a process for hydrocracking and deasphalting resid. In another aspect, embodiments disclosed herein relate to an integrated process for upgrading resid and producing a feed to a residual fluid catalytic cracking (RFCC) unit including multiple hydrocracking stages.
Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are usefill, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil, Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique.
Crude oils range widely in their composition and physical and chemical properties. Heavy crudes are characterized by a relatively high viscosity, low API gravity, and high percentage of high boiling components (i.e., having a normal boiling point typically ranging from about 260° C.(500° F.) to about 600° C.(1112° F.)).
Refined petroleum products generally have higher average hydrogen to carbon ratios on a molecular basis. Therefore, the upgrading of a petroleum refinery hydrocarbon fraction is generally classified into one of two categories: hydrogen addition and carbon rejection. Hydrogen addition is performed by processes such as hydrocracking and hydrotreating. Carbon rejection processes typically produce a stream of rejected high carbon material which may be a liquid or a solid; e.g., coke deposits.
Conventional approaches to upgrade higher boiling materials include converting vacuum residua may be done in numerous ways. In these conventional methods, crude oil is distilled in an atmospheric distillation tower to generate straight run distillates and an atmospheric resid (AR) which may be processed in an atmospheric resid desulfurization (ARDS) unit. The 370+° C. bottoms fraction may be upgraded in a resid fluid catalytic cracking (RFCC) unit to produce distillate fuel products.
In other conventional methods, crude oil may be distilled in an atmospheric distillation tower to generate straight run distillates and an atmospheric resid (AR) which may be further distilled in a vacuum distillation unit to produce vacuum gas oil (VGO) and vacuum resid (VR) streams. The VR may be fed to a vacuum resid desulfurization (VRDS) unit. A VRDS unit is a fixed bed hydrotreating unit where the catalyst requires changeout after a certain interval, typically between 9 and 12 months, The VGO may be fed to an FCC pre-treater to reduce sulfur and nitrogen. The FCC pre-treater effluent and the VRDS 370+° C. unit effluent may be combined and fed to an RFCC unit to produce distillate fuel,
In still other conventional methods, crude oil may be distilled in an atmospheric distillation tower to generate straight run distillates and an atmospheric resid (AR) which may be further distilled in a vacuum distillation unit to produce vacuum gas oil (VGO) and vacuum resid (VR) streams. The VGO may he fed to an FCC pre-treater to reduce sulfur and nitrogen. The VR may he fed to a residue upgrading unit integrated with a fixed-bed hydrotreater/hydrocracker unit to produce distillate fuel products and a byproduct pitch stream,
Conventional hydrocracking processes can be used to upgrade higher boiling materials, such as resid, typically present in heavy crude oil by converting them into more valuable lower boiling materials. For example, at least a portion of the resid feed to a hydrocracking reactor may be converted to a hydrocracking reaction product. The unreacted resid may be recovered from the hydrocracking process and either removed or recycled back to the hydrocracking reactor in order to increase the overall resid conversion.
The resid conversion in a hydrocracking reactor can depend on a variety of flictors, including feedstock composition; the type of reactor used; the reaction severity, including temperature and total pressure conditions; reactor space velocity; hydrogen partial pressure and catalyst type and performance. In particular, the reaction severity may be used to increase the conversion. However, as the reaction severity increases, side reactions may occur inside the hydrocracking reactor to produce various byproducts in the form of coke precursors, sediments, and other deposits as well as byproducts which may form a secondary liquid phase. Excessive formation of such sediments can hinder subsequent processing and can deactivate the hydrocracking catalyst by poisoning, coking, or fouling. Deactivation of the hydrocracking catalyst can not only significantly reduce the resid conversion, but also result in higher catalyst usage, requiring more frequent change-outs of expensive catalyst. Formation of a secondary liquid phase not only deactivates the hydrocracking catalyst, hut also leads to the defluidization of the catalyst bed, thereby limiting the maximum conversion. This leads to formation of “hot zones” within the catalyst bed, exacerbating the formation of coke, which further deactivates the hydrocracking catalyst.
Sediment formation inside the hydrocracking reactor is also a strong function of the feedstock quality. For example, asphaltenes that may be present in the resid feed to the hydrocracking reactor system are especially prone to forming sediments when subjected to severe operating conditions. Thus, separation of the asphaltenes from the resid in order to increase the conversion may be desirable.
One type of process that may be used to remove such asphaltenes from the heavy hydrocarbon residue feed is solvent deasphalting. For example, solvent deasphalting typically involves physically separating the lighter hydrocarbons and the heavier hydrocarbons including asphaltenes based on their relative affinities for the solvent. A light solvent such as a C3 to C7 hydrocarbons can be used to dissolve or suspend the lighter hydrocarbons, commonly referred to as deasphalted oil, allowing the asphaltenes to be precipitated. The two phases are then separated and the solvent is recovered.
Several methods for integrating solvent deasphalting with hydrocracking in order to remove asphaltenes from resid are available. In particular, contacting the residue feed in a solvent deasphalting system to separate the asphaltenes from deasphalted oil is known. The deasphalted oil and the asphaltenes are then each reacted in separate hydrocracking reactor systems.
Moderate overall resid conversions (about 65% to 70%) may be achieved using such processes, as both the deasphalted oil and the asphaltenes are separately hydrocracked. However, the hydrocracking of asphaltenes is at high severity/high conversion, and may present special challenges, as discussed above. For example, operating the asphaltenes hydrocracker at high severity in order to increase the conversion may also cause a high rate of sediment formation, and a high rate of catalyst replacement. In contrast, operating the asphaltenes hydrocracker at low severity will suppress sediment formation, but the per-pass conversion of asphaltenes will be low. In order to achieve a higher overall resid conversion, such processes typically require a high recycle rate of the unreacted resid back to one or more of the hydrocracking reactors. Such high-volume recycle can significantly increase the size of the hydrocracking reactor and/or the upstream solvent deasphalting system.
Petroleum refineries use a number of processing steps to produce the distillate fuel products of gasoline, jet, diesel and distillate fuel oils to meet market demands. In recent times, the product demands for gasoline vs diesel have undergone dramatic shifts and gasoline demand has been increasing relative to diesel demand. Conventional VR hydrocracking systems generally maximize middle distillate production, in particular, diesel. Thus there is a need for refiners who operate ebullated-bed resid hydrocrackers to have the flexibility to readily and economically switch from operating in the max conversion mode which maximizes diesel production to operating in a mode wherein higher quality, i.e., lower S and lower N contents, VGO or VR product is generated which is subsequently processed in a downstream RFCC unit to produce and maximize gasoline production and most importantly, to do so without having to shut down to change out catalysts and thereby suffering loss of product revenues during the shutdown.
Accordingly, there exists a need for improved flexibility resid hydrocracking processes that achieve a high resid conversion, reduces the total number of equipment, reduces the overall equipment size of hydrocracking reactor and/or solvent deasphalter, and require less frequent hydrocracking catalyst change-outs. What would be desired is a process that would take advantage of the ability of a residue hydrocracking process for high conversion and long sustained run lengths without catalyst changeout while achieving the higher quality effluent produced from a fixed bed residue hydrotreating unit, such as ARDS and VRDS. The process should also have the ability for reversible transition.